As far as I know they just used whatever number they wanted, leading to confusing situations where one producers ' 4736 (made up, no idea if this one exists) wo

As far as I know they just used whatever number they wanted, leading to confusing situations where one producers ' 4736 (made up, no idea if this one exists) would be a totally different thing from another's 4736. You'd keep them apart by the manufacturer code, i.e. FOO4736 = 'double-width gizmotron' while BAR4736='confufabulator (4ch)'. I used to leaf through catalogues looking for 'interesting' circuits and came across quite a number of such confusingly-named parts.